Alisa Priddle, Detroit Free Press6:41 p.m. EDT April 17, 2013

DETROIT — Ford marked the production of 1 million Mustangs on Wednesday in Flat Rock, while workers ran two prototypes of the Fusion sedan through the plant’s new body and paint shops.

Flat Rock is a beehive of activity as the plant formerly operated jointly by Mazda and Ford reinvents itself as part of Ford’s $555 million investment.

The plant has about 1,700 employees, including about 200 new hires so far who will be part of the 1,400 on the second shift that will start making Fusions in late August.

But Wednesday was about the Mustang, which was introduced 49 years ago in New York.

To mark the occasion, Raj Nair, head of global product development, rode a ruby red 2014 Mustang convertible off the line while employees joined in the celebration.

Flat Rock has been making the Mustang since 2004. Before that, the pony car was assembled at the company’s Rouge complex in Dearborn.

Nair would not say when the new Mustang will go on sale, but the sixth-generation muscle car will be ready for its 50th anniversary next year. A special edition model could also help mark the milestone.

The new Mustang will be a global vehicle, with some slated for export to Europe. Nair said it will not be changed — other than to meet different regulatory requirements — to meet European tastes.

On Wednesday, workers built the second and third Fusion prototypes, two of 20 test vehicles assembled in a training exercise. Workers look for and fix any bugs in the process. The Fusions are the first cars to come out of a 260,000-square-foot expansion that includes a body shop. Workers also are testing a new paint process.

The next-generation Mustang and Fusion will share the new body shop and paint process, said plant manager Tim Young.

Young said the plant will have the flexibility to adjust the mix of Mustangs and Fusions according to customer demand.

The new body shop has the flexibility to build up to six models. Ford’s longer-term plan is to shift Ford Taurus and Lincoln MKS production to the plant from Chicago. The next-generation of the full-size cars will share underpinnings with the Fusion.

The plant opened in 1987 as a joint venture known as AutoAlliance International. Mazda stopped making vehicles in Michigan in August and Ford assumed full management control at Flat Rock.

The financial unwinding of the joint venture should be complete by mid-2015, Young said